Preparation of pH‐sensitive star‐shaped aliphatic polyesters as precursors of polymersomes

This article reports on the synthesis of a new pH‐sensitive amphiphilic A₂B mikto‐arm star‐shaped aliphatic copolyester [with A = poly(ε‐caprolactone) and B = tertiary amine‐bearing poly(ε‐caprolactone)] with two hydrophobic arms and one hydrophilic arm when protonated at pH = 5.5. First, the ring‐opening polymerization of ε‐caprolactone (εCL) was initiated by an aliphatic diol substituted by an alkyne. The copper(I) catalyzed azide‐alkyne cycloaddition (CuAAC) was use to convert the alkyne into a hydroxyl group prone to initiate the ring‐opening copolymerization of γ‐bromo‐ε‐caprolactone (γBrεCL) and εCL. After the substitution of the bromide atoms into azide functions, the N,N‐dimethylprop‐2‐yn‐1‐amine was grafted onto the azide bearing PCL arm by CuAAC, with the purpose to make the B arm hydrophilic at low pH. The precursors of the A₂B copolymers were characterized by ¹H NMR, SEC, and MALDI‐TOF. As expected, the A₂B copolyester was soluble into water at pH = 5. The formation of polymersomes in water at pH 5 was assessed by DLS and TEM analyses. The effects of the architecture and the molecular weight of the A₂B copolymers on the formation of polymersomes were investigated. Moreover, the versatility of our approach was demonstrated by the synthesis of an AB₂ star‐shaped copolyester. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Norbornene‐functionalized PEO‐b‐PCL: A versatile platform for mikto‐arm star,umbrella‐like,and comb‐like graft copolymers

Well‐defined in‐chain norbornene‐functionalized poly(ethylene oxide)‐b‐poly(?‐caprolactone) copolymers (NB‐PEO‐b‐PCL) were synthesized from a dual clickable containing both hydroxyl‐ and alkyne‐reactive groups, namely heterofunctional norbornene 3‐exo‐(2‐exo‐(hydroxymethyl)norborn‐5‐enyl)methyl hexynoate. A range of NB‐PEO‐b‐PCL copolymers were obtained using a combination of orthogonal organocatalyzed ring‐opening polymerization (ROP) and click copper‐catalyzed azide–alkyne cycloaddition (CuAAC). Ring‐opening metathesis polymerization (ROMP) of NB‐PEO‐b‐PCL macromonomers using ruthenium‐based Grubbs’ catalysts provides comb‐like and umbrella‐like graft copolymers bearing both PEO and PCL grafts on each monomer unit. Mikto‐arm star A₂B₂ copolymers were obtained through a new strategy based on thiol–norbornene photoinitiated click chemistry between 1,3‐propanedithiol and NB‐PEO‐b‐PCL. The results demonstrate that in‐chain NB‐PEO‐b‐PCL copolymers can be used as a platform to prepare mikto‐arm star, umbrella‐, and comb‐like graft copolymers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 4051–4061     

Synthesis of 4μ‐PS2PEO2, 4μ‐PS2PCL2, 4μ‐PI2PEO2, and 4μ‐PI2PCL2 star‐shaped copolymers by the combination of glaser coupling with living anionic polymerization and ring‐opening polymerization

4μ‐A₂B₂ star‐shaped copolymers contained polystyrene (PS), poly(isoprene) (PI), poly(ethylene oxide) (PEO) or poly(ε‐caprolactone) (PCL) arms were synthesized by a combination of Glaser coupling with living anionic polymerization (LAP) and ring‐opening polymerization (ROP). Firstly, the functionalized PS or PI with an alkyne group and a protected hydroxyl group at the same end were synthesized by LAP and then modified by propargyl bromide. Subsequently, the macro‐initiator PS or PI with two active hydroxyl groups at the junction point were synthesized by Glaser coupling in the presence of pyridine/CuBr/N,N,N ′,N ″,N ″‐penta‐methyl diethylenetri‐amine (PMDETA) system and followed by hydrolysis of protected hydroxyl groups. Finally, the ROP of EO and ε‐CL monomers was carried out using diphenylmethyl potassium (DPMK) and tin(II)‐bis(2‐ethylhexanoate) (Sn(Oct)₂) as catalyst for target star‐shaped copolymers, respectively. These copolymers and their intermediates were well characterized by SEC, ¹H NMR, MALDI‐TOF mass spectra and FT‐IR in details. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Synthesis of Star‐Shaped Poly(ϵ‐caprolactone)‐b‐Poly(styrene) Block Copolymer by Combining Ring‐Opening Polymerization and Atom Transfer Radical Polymerization

Newly designed star‐shaped block copolymers made of poly(?‐caprolactone) (PCL) and polystyrene (PS) were synthesized by combining ring‐opening polymerization (ROP) of ?‐caprolactone (CL) and atom transfer radical polymerization (ATRP) of styrene (St). The switch from the first to the second mechanism was obtained by selective transformation of “living” radical sites. First, tri‐ and tetrafunctional initiators were used as an initiator for the “living” ring opening polymerization (ROP) of ?‐caprolactone producing a hydroxyl terminated three or four arm star‐shaped polymer. Next, the OH end groups of PCL star branches were derivatized into 2‐bromoisobutyrate groups which gave rise to the corresponding tri‐ and tetrabromoester ended‐PCL stars; the latter served as macroinitiators for the ATRP of styrene at 110°C in the presence of CuBr/2,2‐bipyridine (Bipy) catalyst system affording star‐shaped block copolymers PCL_n‐b‐PS_n (n=3 or 4). The samples obtained were characterizated by ¹H‐NMR spectroscopy and GPC (gel permeation chromatograph). These copolymers exhibited the expected structure. The crystallization of star‐shaped block copolymers was studied by DSC (differential scanning calorimetry). The results show that when the content of the PS block increased, the T_m of the star‐shaped block copolymer decreased.     

Three‐arm star block copolymers of ε‐caprolactone and acrylic acid: Synthesis and micellization behavior

A series of well‐defined three‐arm star poly(ε‐caprolactone)‐b‐poly(acrylic acid) copolymers having different block lengths were synthesized via the combination of ring‐opening polymerization (ROP) and atom transfer radical polymerization (ATRP). First, three‐arm star poly(ε‐caprolactone) (PCL) (M_n = 2490–7830 g mol^?1; M_w/M_n = 1.19–1.24) were synthesized via ROP of ε‐caprolactone (ε‐CL) using tris(2‐hydroxyethyl)cynuric acid as three‐arm initiator and stannous octoate (Sn(Oct)₂) as a catalyst. Subsequently, the three‐arm macroinitiator transformed from such PCL in high conversion initiated ATRPs of tert‐butyl acrylate (^tBuA) to construct three‐arm star PCL‐b‐P^tBuA copolymers (M_n = 10,900–19,570 g mol^?1; M_w/M_n = 1.14–1.23). Finally, the three‐arm star PCL‐b‐PAA copolymer was obtained via the hydrolysis of the PtBuA segment in three‐arm star PCL‐b‐P^tBuA copolymers. The chain structures of all the polymers were characterized by gel permeation chromatography, proton nuclear magnetic resonance (¹H NMR), and Fourier transform infrared spectroscopy. The aggregates of three‐arm star PCL‐b‐PAA copolymer were studied by the determination of critical micelles concentration and transmission electron microscope. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Synthesis,characterization, and fluorescence of pyrene‐containing eight‐arm star‐shaped dendrimer‐like copolymer with pentaerythritol core

Novel and well‐defined pyrene‐containing eight‐arm star‐shaped dendrimer‐like copolymers were successfully achieved by combination of esterification, atom transfer radical polymerization (ATRP), divergent reaction, ring‐opening polymerization (ROP), and coupling reaction on the basis of pentaerythritol. The reaction of pentaerythritol with 2‐bromopropionyl bromide permitted ATRP of styrene (St) to form four‐arm star‐shaped polymer (PSt‐Br)₄. The molecular weights of these polymers could be adjusted by the variation of monomer conversion. Eight‐hydroxyl star‐shaped polymer (PSt‐(OH)₂)₄ was produced by the divergent reaction of (PSt‐Br)₄ with diethanolamine. (PSt‐(OH)₂)₄ was used as the initiator for ROP of ε‐caprolactone (CL) to produce eight‐arm star‐shaped dendrimer‐like copolymer (PSt‐b‐(PCL)₂)₄. The molecular weights of (PSt‐b‐(PCL)₂)₄ increased linearly with the increase of monomer. After the coupling reaction of hydroxyl‐terminated (PSt‐b‐(PCL)₂)₄ with 1‐pyrenebutyric acid, pyrene‐containing eight‐arm star‐shaped dendrimer‐like copolymer (PSt‐b‐(PCL‐pyrene)₂)₄ was obtained. The eight‐arm star‐shaped dendrimer‐like copolymers presented unique thermal properties and crystalline morphologies, which were different from those of linear poly(ε‐caprolactone) (PCL). Fluorescence analysis indicated that (PSt‐b‐(PCL‐pyrene)₂)₄ presented slightly stronger fluorescence intensity than 1‐pyrenebutyric acid when the pyrene concentration of them was the same. The obtained pyrene‐containing eight‐arm star‐shaped dendrimer‐like copolymer has potential applications in biological fluorescent probe, photodynamic therapy, and optoelectronic devices. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2788–2798, 2008     

Novel synthesis of biodegradable amphiphilic linear and star block copolymers based on poly(ε‐caprolactone) and poly(ethylene glycol)

Biodegradable, amphiphilic, diblock poly(ε‐caprolactone)‐block‐poly(ethylene glycol) (PCL‐b‐PEG), triblock poly(ε‐caprolactone)‐block‐poly(ethylene glycol)‐block‐poly(ε‐caprolactone) (PCL‐b‐PEG‐b‐PCL), and star shaped copolymers were synthesized by ring opening polymerization of ε‐caprolactone in the presence of poly(ethylene glycol) methyl ether or poly(ethylene glycol) or star poly(ethylene glycol) and potassium hexamethyldisilazide as a catalyst. Polymerizations were carried out in toluene at room temperature to yield monomodal polymers of controlled molecular weight. The chemical structure of the copolymers was investigated by ¹H and ¹³C NMR. The formation of block copolymers was confirmed by ¹³C NMR and DSC investigations. The effects of copolymer composition and molecular structure on the physical properties were investigated by GPC and DSC. For the same PCL chain length, the materials obtained in the case of linear copolymers are viscous whereas in the case of star copolymer solid materials are obtained with low T_g and T_m temperatures. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3975–3985, 2007     

Synthesis and self‐assembly of well‐defined cyclodextrin‐centered amphiphilic A14B7 multimiktoarm star copolymers based on poly(ε‐caprolactone) and poly(acrylic acid)

Novel amphiphilic A₁₄B₇ multimiktoarm star copolymers composed of 14 poly(ε‐caprolactone) (PCL) arms and 7 poly(acrylic acid) (PAA) arms with β‐cyclodextrin (β‐CD) as core moiety were synthesized by the combination of controlled ring‐opening polymerization (CROP) and atom transfer radical polymerization (ATRP). 14‐Arm star PCL homopolymers (CDSi‐SPCL) were first synthesized by the CROP of CL using per‐6‐(tert‐butyldimethylsilyl)‐β‐CD as the multifunctional initiator in the presence of Sn(Oct)₂ at 125 °C. Subsequently, the hydroxyl end groups of CDSi‐SPCL were blocked by acetyl chloride. After desilylation of the tert‐butyldimethylsilyl ether groups from the β‐CD core, 7 ATRP initiating sites were introduced by treating with 2‐bromoisobutyryl bromide, which further initiated ATRP of tert‐butyl acrylate (tBA) to prepare well‐defined A₁₄B₇ multimiktoarm star copolymers [CDS(PCL‐PtBA)]. Their molecular structures and physical properties were in detail characterized by ¹H NMR, SEC‐MALLS, and DSC. The selective hydrolysis of tert‐butyl ester groups of the PtBA block gave the amphiphilic A₁₄B₇ multimiktoarm star copolymers [CDS(PCL‐PAA)]. These amphiphilic copolymers could self‐assemble into multimorphological aggregates in aqueous solution, which were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2961–2974, 2010     

Synthesis of amphiphilic A4B4 star‐shaped copolymers by mechanisms transformation combining with thiol‐ene reaction

Using core‐first strategy, the amphiphilic A₄B₄ star‐shaped copolymers [poly(ethylene oxide)]₄[poly(ε‐caprolactone)]₄ [(PEO)₄(PCL)₄], [poly(ethylene oxide)]₄[poly(styrene)]₄ [(PEO)₄(PS)₄], and [poly(ethylene oxide)]₄[poly(tert‐butyl acrylate)]₄ [(PEO)₄(PtBA)₄] were synthesized by mechanisms transformation combining with thiol‐ene reaction. First, using a designed multifunctional mikto‐initiator with four active hydroxyl groups and four allyl groups, the four‐armed star‐shaped polymers (PEO‐Ph)₄/(OH)₄ with four active hydroxyl groups at core position were obtained by sequential ring‐opening polymerization (ROP) of ethylene oxide monomers, capping reaction of living oxyanion with benzyl chloride, and transformation of allyl groups into hydroxyl groups by thiol‐ene reaction. Then, the A₄B₄ star‐shaped copolymers (PEO)₄(PS)₄ or (PEO)₄(PtBA)₄ were obtained by atom transfer radical polymerization (ATRP) of styrene or tert‐butyl acrylate (tBA) monomers from macroinitiator of (PEO‐Ph)₄/(Br)₄, which was obtained by esterification of (PEO‐Ph)₄/(OH)₄ with 2‐bromoisobutyryl bromide. The A₄B₄ star‐shaped copolymers (PEO)₄(PCL)₄ were also obtained by ROP of ε‐caprolactopne monomers from macroinitiator of (PEO‐Ph)₄/(OH)₄. The target copolymers and intermediates were characterized by size‐exclusion chromatography, matrix‐assisted laser desorption/ionization time‐of‐flight mass spectroscopy, and nuclear magnetic resonance in detail. This synthetic route might be a versatile one to various A_nB_n (n ≥ 3) star‐shaped copolymers with defined structure and compositions. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4572–4583     

Synthesis and characterization of well‐defined cyclodextrin‐centered seven‐arm star poly(ε‐caprolactone)s and amphiphilic star poly(ε‐caprolactone‐b‐ethylene glycol)s

Per‐2,3‐acetyl‐β‐cyclodextrin with seven primary hydroxyl groups was synthesized by selective modification and used as multifunctional initiator for the ring‐opening polymerization of ε‐caprolactone (CL). Well‐defined β‐cyclodextrin‐centered seven‐arm star poly(ε‐caprolactone)s (CDSPCLs) with narrow molecular weight distributions (≤1.15) have been successfully prepared in the presence of Sn(Oct)₂ at 120 °C. The molecular weight of CDSPCLs was characterized by end group ¹H NMR analyses and size‐exclusion chromatography (SEC), which could be well controlled by the molar ratio of the monomer to the initiator. Furthermore, amphiphilic seven‐arm star poly(ε‐caprolactone‐b‐ethylene glycol)s (CDSPCL‐b‐PEGs) were synthesized by the coupling reaction of CDSPCLs with carboxyl‐terminated mPEGs. ¹H NMR and SEC analyses confirmed the expected star block structures. Differential scanning calorimetry analyses suggested that the melting temperature (T_m), the crystallization temperature (T_c), and the crystallinity degree (X_c) of CDSPCLs all increased with the increasing of the molecular weight, and were lower than that of the linear poly(ε‐caprolactone). As for CDSPCL‐b‐PEGs, the T_c and T_m of the PCL blocks were significantly influenced by the PEG segments in the copolymers. Moreover, these amphiphilic star block copolymers could self‐assemble into spherical micelles with the particle size ranging from 10 to 40 nm. Their micellization behaviors were characterized by dynamic light scattering and transmission electron microscopy. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6455–6465, 2008     

A convenient synthesis of α,α′‐ homo‐ and α,α′‐hetero‐bifunctionalized poly(ε‐caprolactone)s by ring opening polymerization: The potentially valuable precursors for miktoarm star copolymers

Two new ring opening polymerization (ROP) initiators, namely, (3‐allyl‐2‐(allyloxy)phenyl)methanol and (3‐allyl‐2‐(prop‐2‐yn‐1‐yloxy)phenyl)methanol each containing two reactive functionalities viz. allyl, allyloxy and allyl, propargyloxy, respectively, were synthesized from 3‐allylsalicyaldehyde as a starting material. Well defined α‐allyl, α′‐allyloxy and α‐allyl, α′‐propargyloxy bifunctionalized poly(ε‐caprolactone)s with molecular weights in the range 4200–9500 and 3600–10,900 g/mol and molecular weight distributions in the range 1.16–1.18 and 1.15–1.16, respectively, were synthesized by ROP of ε‐caprolactone employing these initiators. The presence of α‐allyl, α′‐allyloxy and α‐allyl, α′‐propargyloxy functionalities on poly(ε‐caprolactone)s was confirmed by FT‐IR, ¹H, ¹³C NMR spectroscopy, and MALDI‐TOF analysis. The kinetic study of ROP of ε‐caprolactone with both the initiators revealed the pseudo first order kinetics with respect to ε‐caprolactone consumption and controlled behavior of polymerization reactions. The usefulness of α‐allyl, α′‐allyloxy functionalities on poly(ε‐caprolactone) was demonstrated by performing the thiol‐ene reaction with poly(ethylene glycol) thiol to obtain (mPEG)₂‐PCL miktoarm star copolymer. α‐Allyl, α′‐propargyloxy functionalities on poly(ε‐caprolactone) were utilized in orthogonal reactions i.e copper catalyzed alkyne‐azide click (CuAAC) with azido functionalized poly(N‐isopropylacrylamide) followed by thiol‐ene reaction with poly(ethylene glycol) thiol to synthesize PCL‐PNIPAAm‐mPEG miktoarm star terpolymer. The preliminary characterization of A₂B and ABC miktoarm star copolymers was carried out by ¹H NMR spectroscopy and gel permeation chromatography (GPC). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 844–860     

Synthesis and Characterization of Amphiphilic Poly(caprolactone) Star Block Copolymers

Summary: Novel, star‐shaped, amphiphilic block copolymers composed of fully degradable poly(caprolactone) were synthesized by sequential addition polymerization. In the first step, four‐arm macroinitiators were produced by ring‐opening polymerization of caprolactone by initiation with pentaerythritol. Then, block copolymers were synthesized by sequential addition of 4‐(2‐benzyloxyethyl)‐ε‐caprolactone to the four‐arm macroinitiators. Star‐shaped, amphiphilic block copolymers containing poly(caprolactone)‐block‐poly[4‐(2‐hydroxyethyl)caprolactone] segments were obtained by catalytic debenzylation.

Four‐arm amphiphilic polycaprolactone star block copolymer.     

Synthesis,characterization, and thermal properties of dendrimer‐star,block‐comb copolymers by ring‐opening polymerization and atom transfer radical polymerization

Novel and well‐defined dendrimer‐star, block‐comb polymers were successfully achieved by the combination of living ring‐opening polymerization and atom transfer radical polymerization on the basis of a dendrimer polyester. Star‐shaped dendrimer poly(?‐caprolactone)s were synthesized by the bulk polymerization of ?‐caprolactone with a dendrimer initiator and tin 2‐ethylhexanoate as a catalyst. The molecular weights of the dendrimer poly(?‐caprolactone)s increased linearly with an increase in the monomer. The dendrimer poly(?‐caprolactone)s were converted into macroinitiators via esterification with 2‐bromopropionyl bromide. The star‐block copolymer dendrimer poly(?‐caprolactone)‐block‐poly(2‐hydroxyethyl methacrylate) was obtained by the atom transfer radical polymerization of 2‐hydroxyethyl methacrylate. The molecular weights of these copolymers were adjusted by the variation of the monomer conversion. Then, dendrimer‐star, block‐comb copolymers were prepared with poly(L ‐lactide) blocks grafted from poly(2‐hydroxyethyl methacrylate) blocks by the ring‐opening polymerization of L ‐lactide. The unique and well‐defined structure of these copolymers presented thermal properties that were different from those of linear poly(?‐caprolactone). © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6575–6586, 2006     

Encapsulation and release by star‐shaped block copolymers as unimolecular nanocontainers

A five‐arm star‐shaped poly(ethylene oxide) (PEO) with terminal bromide groups was used as a macroinitiator for the atom transfer radical polymerization of tert‐butyl acrylate (tBA), resulting in five‐arm star‐shaped poly(ethylene oxide)‐block‐poly(tert‐butyl acrylate) block copolymers. The polymerization proceeded in a controlled way using a copper(I)bromide/pentamethyl diethylenetriamine catalytic system in acetonitrile as solvent. The hydrolysis of the tBA blocks of the amphiphilic star‐shaped PEO‐b‐PtBA block copolymer resulted in dihydrophilic star structures. The encapsulation of the star‐block copolymers and their release properties in acid environment have been followed by UV‐spectroscopy and color changes, using the dye methyl orange as a hydrophilic guest molecule. Characterization of the structures has been done by ¹H NMR, size exclusion chromatography, MALDI‐TOF, and differential scanning calorimetry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 650–660, 2008     

Morphology and microphase separation of star copolymers

Star copolymers have attracted significant interest due to their different characteristics compared with diblock copolymers, including higher critical micelle concentration, lower viscosity, unique spatial shape, or morphologies. Development of synthetic skills such as anionic polymerization and controlled radical polymerization have made it possible to make diverse architectures of polymers. Depending on the molecular architecture of the copolymer, numerous morphologies are possible, for instance, Archimedean tiling patterns and cylindrical microdomains at symmetric volume fraction for miktoarm star copolymers as well as asymmetric lamellar microdomains for star‐shaped copolymers, which have not been reported for linear block copolymers. In this review, we focus on morphologies and microphase separations of miktoarm (A_mB_n and ABC miktoarm) star copolymers and star‐shaped [(A‐b‐B)_n] copolymers with nonlinear architecture. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1–21     

Novel synthesis of biodegradable star poly(ethylene glycol)‐block‐poly(lactide) copolymers

Biodegradable star‐shaped poly(ethylene glycol)‐block‐poly(lactide) copolymers were synthesized by ring‐opening polymerization of lactide, using star poly(ethylene glycol) as an initiator and potassium hexamethyldisilazide as a catalyst. Polymerizations were carried out in toluene at room temperature. Two series of three‐ and four‐armed PEG‐PLA copolymers were synthesized and characterized by gel permeation chromatography (GPC) as well as ¹H and ¹³C NMR spectroscopy. The polymerization under the used conditions is very fast, yielding copolymers of controlled molecular weight and tailored molecular architecture. The chemical structure of the copolymers investigated by ¹H and ¹³C NMR indicates the formation of block copolymers. The monomodal profile of molecular weight distribution by GPC provided further evidence of controlled and defined star‐shaped copolymers as well as the absence of cyclic oligomeric species. The effects of copolymer composition and lactide stereochemistry on the physical properties were investigated by GPC and differential scanning calorimetry. For the same PLA chain length, the materials obtained in the case of linear copolymers are more viscous, whereas in the case of star copolymer, solid materials are obtained with reduction in their T_g and T_m temperatures. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3966–3974, 2007     

Synthesis and crystallization of star‐shaped photocleavable poly(ε‐caprolactone)s

Here, we report on the synthesis and different crystallization behavior of linear‐ and star‐ PCL's containing a photocleavable linker (5‐hydroxy‐2‐nitro benzaldehyde), modulated by photochemical switching. Basis is the attachment of a photocleavable moiety close to the star‐core of a three‐arm star poly(caprolactone), so that the crystallization behavior can be controlled via a photochemical stimulus. The polymerization of ε‐caprolactone using a trivalent photocleavable initiator and stannous octanoate catalyst resulted in the synthesis of different molecular weights of star‐shaped photocleavable polymers. Various techniques like ¹H NMR and ESI‐TOF‐MS confirmed the successful synthesis of the star‐shaped polymers. Complete photocleavage is ensured via GPC, HPLC, and ESI‐TOF‐MS. DSC studies clearly indicated the enhancement in crystallinity after photocleavage of the star‐shaped poly(ε‐caprolactone)s. Hence, for the first time phototriggered crystallization behavior of PCL polymers is reported, where the confinement exerted by the star architecture is removed by photoirradiation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 642–649     

Synthesis and characterization of resorcinarene‐centered amphiphilic A8B4 miktoarm star copolymers based on poly(ε‐caprolactone) and poly(ethylene glycol) by combination of CROP and “click” chemistry

Well‐defined amphiphilic A₈B₄ miktoarm star copolymers with eight poly(ethylene glycol) chains and four poly(ε‐caprolactone) arms (R‐8PEG‐4PCL) were prepared using “click” reaction strategy and controlled ring‐opening polymerization (CROP). First, multi‐functional precursor (R‐8N₃‐4OH) with eight azides and four hydroxyls was synthesized based on the derivatization of resorcinarene. Then eight‐PEG‐arm star polymer (R‐8PEG‐4OH) was prepared through “click” reaction of R‐8N₃‐4OH with pre‐synthesized alkyne‐terminated monomethyl PEG (mPEG‐A) in the presence of CuBr/N,N,N′,N″,N″′‐ pentamethyldiethylenetriamine (PMDETA) in DMF. Finally, R‐8PEG‐4OH was used as tetrafunctional macroinitiator to prepare resorcinarene‐centered A₈B₄ miktoarm star copolymers via CROP of ε‐caprolactone utilizing Sn(Oct)₂ as catalyst at 100 °C. These miktoarm star copolymers could self‐assemble into spherical micelles in aqueous solution with resorcinarene moieties on the hydrophobic/hydrophilic interface, and the particle sizes could be controlled by the ratio of PCL to PEG. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2824–2833.     

Thermo‐responsive fluorescent micelles from amphiphilic A3B miktoarm star copolymers prepared via a combination of SET‐LRP and RAFT polymerization

A novel amphiphilic A₃B miktoarm star copolymer poly(N‐isopropylacrylamide)₃‐poly(N‐vinylcarbazole) ((PNIPAAM)₃(PVK)) was successfully synthesized by a combination of single‐electron transfer living radical polymerization (SET‐LRP) and reversible addition‐fragmentation chain transfer (RAFT) polymerization. First, the well‐defined three‐armed poly(N‐isopropylacrylamide) (PNIPAAM)₃ was prepared via SET‐LRP of N‐isopropylacrylamide in acetone at 25 °C using a tetrafunctional bromoxanthate iniferter (Xanthate‐Br₃) as the initiator and Cu(0)/PMDETA as a catalyst system. Secondly, the target amphiphilic A₃B miktoarm star copolymer ((PNIPAAM)₃(PVK)) was prepared via RAFT polymerization of N‐vinylcarbazole (NVC) employing (PNIPAAM)₃ as the macro‐RAFT agent. The architecture of the amphiphilic A₃B miktoarm star copolymers were characterized by GPC, ¹H‐NMR spectra. Furthermore, the fluorescence intensity of micelle increased with the temperature and had a good temperature reversibility, which was investigated by dynamic light scattering (DLS), fluorescent and UV‐vis spectra. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4268–4278, 2010     

An efficient synthetic route to well‐defined theta‐shaped copolymers

A series of well‐defined θ‐shaped copolymers composed of polystyrene (PS) and poly(ε‐caprolactone) (PCL) with controlled molecular weight and narrow molecular weight distribution have been successfully synthesized without any purification procedure by the combination of atom transfer radical polymerization (ATRP), ring‐opening polymerization (ROP), and the “click” chemistry. The synthetic process involves two steps: (1) synthesis of AB₂ miktoarm star copolymers, which contain one PCL chain terminated with two acetylene groups and two PS chains with two azido groups at their one end, (α,α′‐diacetylene‐PCL) (ω‐azido‐PS)₂, by ROP, ATRP, and the terminal group transformation; (2) intramolecular cyclization of AB₂ miktoarm star copolymers to produce well‐defined pure θ‐shaped copolymers using “click” chemistry under high dilution. The ¹H NMR, FTIR, and gel permeation chromatography techniques were applied to characterize the chemical structures of the resultant intermediates and the target polymers. Their thermal behavior was investigated by DSC. The mobility decrease of PCL chain across PS ring in the theta‐shaped copolymers restricts the crystallization ability of PCL segment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2620–2630, 2009